Electrodes are widely used in many devices that store electrical energy, including primary (non-rechargeable) battery cells, secondary battery cells, fuel cells, and capacitors. In some applications, it is necessary or desirable to keep the electrodes separated so that the electrodes do not come into direct electrical contact with each other, while at the same time allowing some chemical, electrical, or other kind of interaction between the electrodes. One such application that uses electrodes is in double layer capacitors, also known as electrochemical capacitors, supercapacitors, and ultracapacitors.
Double layer capacitors employ, as their energy storage elements, electrodes immersed in an electrolytic solution (electrolyte). A porous separator impregnated with the electrolyte ensures that the electrodes do not come in contact with each other. A double layer of charges is formed at each interface between the solid electrodes and the electrolyte. Double layer capacitors owe their descriptive name to these layers.
When electric potential is applied between a pair of electrodes, ions that exist within the electrolyte are attracted to the surfaces of the electrodes, and migrate towards the electrodes. A layer of oppositely-charged ions is thus created and maintained near each electrode surface. The electrical energy is stored in the charge separation layers between the ionic layers and the charge layers of the corresponding electrode surfaces. The charge separation layers behave essentially as capacitors.
In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for this volumetric and weight efficiency. First, the width of the charge separation layers is very small, on the order of nanometers. Second, the electrodes may be made from a porous material, having very large effective surface area per unit volume, i.e., very large normalized effective surface area. Because capacitance is directly proportional to the electrode area and inversely proportional to the width of the charge separation layer, the combined effect of the narrow charge separation layer and large surface area results in capacitance that is very high in comparison to that of conventional capacitors of similar size. High capacitance enables double layer capacitors to receive, store, and release large supplies of electrical energy.
Another important performance parameter of a capacitor is its internal resistance, also known as equivalent series resistance or “ESR.” Frequency response of a capacitor depends on the characteristic time constant of the capacitor, which is essentially a product of the capacitance and the capacitor's equivalent series resistance, or “RC.” To put it differently, equivalent series resistance limits both charge and discharge rates of a capacitor, because the resistance limits the current that flows into or out of the capacitor. Maximizing the charge and discharge rates is important in many applications. In automotive applications, for example, a capacitor used as the energy storage element powering a vehicle's engine has to be able to provide high instantaneous power during acceleration, and to receive bursts of power produced by regenerative braking. In internal combustion vehicles, the capacitor periodically powers a vehicle's starter, also requiring high power output in relation to the size of the capacitor.
The internal resistance also creates heat during both charge and discharge cycles. Heat causes mechanical stresses and speeds up various chemical reactions, thereby accelerating capacitor aging. Moreover, the energy converted into heat is lost, decreasing the efficiency of the capacitor. It is therefore desirable to reduce internal resistance of capacitors.
Active materials used for electrode construction—activated carbon, for example—may have limited specific conductance. Thus, large contact area may be desired to minimize the interfacial contact resistance between the electrode and its terminal. The active material may also be too brittle or otherwise unsuitable for directly connecting to capacitor terminals. Additionally, the material may have relatively low tensile strength, needing mechanical support in some applications. For these reasons, electrodes often incorporate current collectors.
A current collector is typically a sheet of conductive material to which the active electrode material is attached. Aluminum foil is commonly used as the current collector element of an electrode. In one electrode fabrication process, for example, a film that includes activated carbon powder is produced, and then attached to a thin aluminum foil using an adhesive layer. To improve the quality of the interfacial bond between the film of active electrode material and the current collector, the combination of the film and the current collector is processed in a pressure laminator, for example, a calender or another nip. Pressure lamination increases the bonding forces between the film and the current collector, and reduces the equivalent series resistance of the energy storage device that employs the electrode.
As has already been mentioned, a porous separator keeps the electrodes of a double layer capacitor from coming into direct electrical contact with each other. In one double layer capacitor fabrication process, electrodes are made using techniques known in the art, with each electrode having an active electrode layer attached to a current collector. The separator is then sandwiched between two electrodes so that the separator is in contact with the layers of active electrode material of the two electrodes. The electrode-separator-electrode assembly may then be jelly-rolled, equipped with a pair of terminals, and placed in a can or another container holding electrolyte.
The manufacture of electrodes before attachment to separator may not be advantageous. For example, an active electrode layer may be damaged in the process of calendering and laminating it to a current collector. Similarly, when current collector is deposited on an active electrode layer using certain metallization techniques such as arc spraying, the active electrode layer may be damaged from the physical forces and high temperatures created by the metallization process. To avoid such damage, appropriate support backing may need to be supplied to one surface of the active electrode layer prior to attaching the current collector to the opposite surface of the layer.
These and other disadvantages are addressed by embodiments of the present invention, which are described in the following disclosure.